Air travelers have long sought the convenience and efficiency of widespread supersonic commercial aviation only to be denied by technological, economic, and political roadblocks. With operations spanning over a quarter of a century, the Concorde remains the only commercial aircraft that travels at supersonic speeds but struggles with technological obsolescence. Fuel consumption and maintenance requirements of the Concorde strain commercial feasibility in today's competitive environment. Possibly overshadowing other technological and economic shortcomings is the Concorde's thunderous sonic boom that is highly annoying due to its perceived loudness and startle, a burden that restricts the Concorde's supersonic operations to primarily oceanic routes.
The sonic boom imposes many practical limitations for commercial supersonic aviation as annoyance with sonic boom loudness and startle results in the prohibition of commercial supersonic aircraft operations over most populated landmasses.
A sonic boom occurs due to pressure waves that form when an aircraft moves at supersonic speeds. As the aircraft approaches supersonic speeds, air at the leading edge of the configuration compresses to a non-linear threshold where discontinuities in flow properties, manifest through a pressure pulse and propagated through the atmosphere. Pressure pulse intensity decreases as a consequence of propagation through the atmosphere and changes shape into an N-shaped wave within which pressure rises sharply, gradually declines, and then rapidly returns to ambient atmospheric pressure. A wall of compressed air that moves at aircraft speed spreads from the wave and, in passing over ground is heard and felt as a sonic boom. Rapid changes in pressure at the beginning and end of the N-wave produce the signature double bang of the sonic boom.
Research has demonstrated that boom intensity can be reduced by altering aircraft shape, length, and weight. An aircraft that is long in proportion to weight spreads the overpressure across a greater distance, resulting in a lower peak pressure. Furthermore, wings that are spread along the body and not concentrated in the center as in a conventional aircraft have a greater lifting length and produce a pressure pulse that is similarly spread, resulting in a smaller sonic boom.
One technique for boom reduction is shaping. Shaping alters source pressure disturbance such that a non-N-wave shape is imposed on the ground. Shaping can reduce loudness by 15-20 dB or more with no added energy beyond that to sustain flight. Minimizing loudness is based on insight regarding changes in aircraft pressure disturbances during propagation to the ground. During the sixties and seventies, Jones, Seebass, George, and Darden developed a practical analytical guideline for low boom design.
Studies have shown that sonic boom loudness at audible frequencies correlates with annoyance. Therefore supersonic over land flight could only be achieved by reducing the sonic boom to acceptable sound levels. Shaped sonic booms are only achieved deliberately. No existing aircraft creates a shaped sonic boom that persists for more than fraction of the distance to the ground while flying at an efficient cruise altitude, since non-shaped pressure distributions quickly coalesce into the fundamental N-wave shape. Audible frequencies for a sonic boom occur wherever pressure changes rapidly, essentially at the beginning and end of a typical N-waveform. Shocks become quieter at decreasing magnitudes and increasing rise times of the pressure change. The N-wave form generates the largest possible shock magnitude from a particular disturbance. On average the front of a supersonic aircraft generates an increase from ambient pressure while the rear generates a decrease in pressure. The disturbances stretch and also coalesce because shocks travel at speeds that vary in proportion to the magnitudes of the local pressure. Higher pressures travel faster moving forward and coalescing into a single front shock, and likewise, the aft low pressures coalesce into a single rear shock. Variation in propagation speed stretches the disturbance during propagation to the ground to two to three times the vehicle length—-very significant stretching. Shaped boom techniques typically attempt to prevent coalescing of the pressure disturbance by adding a large compression at the aircraft nose and an expansion at the tail with pressure between constrained to very weak compression and expansion, with correspondingly slow coalescence speeds. The shaped boom stretches the ends of the signature faster than the in-between pressures, stretching without coalescing and creating a non-N-wave sonic boom at the ground. The vehicle's pressure distribution is constrained to this particular George-Seebass-Darden shape that produces the minimum shock strength possible through the least coalescence possible.